1. Field of the Invention
The present invention relates to a holding apparatus configured to hold an optical element.
2. Description of the Related Art
A holding apparatus configured to hold an optical element is used in apparatuses such as a semiconductor exposure apparatus.
The semiconductor exposure apparatus is an apparatus used in forming a circuit by transferring a pattern of a reticle on a silicon wafer. In order to form a highly-integrated circuit, it is necessary to improve overlay accuracy of multiple patterns which are transferred onto the silicon wafer.
In order to improve the overlay accuracy, it is necessary to reduce alignment error, magnification error, and image distortion. The alignment error can be reduced by adjusting a relative position of the reticle and the wafer. The magnification error can be reduced by moving a part of an optical element, which constitutes a portion of a projection optical system, in the optical axis direction.
When the optical element is moved in the optical axis direction, error components having directions other than the optical axis, particularly, parallel decentering and tilt error, need to be controlled so that they do not increase. The image distortion can be reduced by parallel-decentering or tilt-decentering a portion of the optical element, which constitutes the projection optical system.
Under the above circumstances, a holding apparatus having amovement mechanism for an optical element, which allows an improvement in the overlay accuracy, is drawing attention. For example, Japanese Patent Application Laid-Open No. 2001-343575 discusses such a holding apparatus. FIG. 14 illustrates a configuration of the holding apparatus discussed in Japanese Patent Application Laid-Open No. 2001-343575.
In FIG. 14, a movable lens 38a is supported by a plurality of receiving seats, which protrude from an inner circumference of a first lens cell 46, and fixed to the first lens cell 46 by a lens pressing member or the like. The first lens cell 46 is fixed to an inner ring portion 44a. The inner ring portion 44a is driven in the optical axis direction by an actuator 50 via a link arm 59.
Three actuators 50 are provided at equal angular intervals along the periphery of an outer ring portion 44b located lateral to the inner ring portion 44a. Further, a sensor 72 is provided between the actuators 50. The sensor 72 measures the position of the inner ring portion 44a in relation to the outer ring portion 44b. The number of sensors 72 is three and each of the sensors 72 can measure a relative amount of displacement of the inner ring portion 44a to the outer ring portion 44b in the optical axis direction.
FIG. 15 illustrates a holding apparatus discussed in Japanese Patent Application Laid-Open No. 10-054932.
In a projection optical system 10 illustrated in FIG. 15, each of a plurality of lens elements 2a is held by an annular lens frame. Each lens frame is supported by an inner protruding portion of lens barrels 62a, 62b, and 62c. Further, actuators 60b and 60c configured to drive the lens barrels 62a, 62b, and 62c and displacement detectors 64a and 64b configured to detect displacement between the lens barrels 62a, 62b, and 62c are mounted on outer protruding portions 36a, 36b, and 36c of the lens barrels 62a, 62b, and 62c. 
According to the configuration discussed in Japanese Patent Application Laid-Open No. 2001-343575, when the inner ring portion 44a is tilted, the inner ring portion 44a may be deformed. This deformation is caused by an insufficient margin in rotation direction at a coupling portion between the inner ring portion 44a and the actuator 50, which mainly drives the inner ring portion 44a. Then, the deformation of the inner ring portion 44a may have an adverse effect on a result of detection of an amount of tilt by the sensor 72, so that the sensor 72 cannot accurately detect a tilt of the inner ring portion 44a. Thus, a deviation in tilt may occur between the movable lens 38a and the inner ring portion 44a. 
In the holding apparatus discussed in Japanese Patent Application Laid-Open No. 10-054932, the displacement detectors 64a and 64b detect displacement between the lens barrels 62a, 62b, and 62c, and the actuators 60b and 60c drive the lens barrels 62a, 62b, and 62c. Accordingly, it is difficult to accurately position the lens barrels 62a, 62b, and 62c, which are relatively heavy.